User blog comment:PokeFreak117/UPDATE/@comment-24553128-20170910014429

Hunt for DeepseaScales on blue-striped Basculins. Then, trade the scales for tickets to islands to catch the three legendary birds.

Moltres with the Obsidian Ticket

Zapdos with Voltiac Ticket

Articuno with Frigid Ticket

ONE DeepseaScale for each ticket.

Hope this helps :)